1. Field of the Invention
The present invention relates to matching circuits used in amplifiers and the like and to power amplifiers. More specifically, the present invention relates to a low-loss multiband matching circuit that can establish matching between the input-output impedance of a circuit element having frequency characteristics such as an amplification device and the characteristic impedance of a peripheral circuit in a plurality of frequency bands and to a multiband power amplifier having the low-loss multiband matching circuit.
2. Description of the Related Art
One requirement of radio equipment is the capability of dealing with signals in a plurality of frequency bands (multiband). An indispensable component included in radio equipment is a power amplifier. Something that is needed for efficient amplification is a multiband matching circuit that can establish impedance matching between the input impedance ZI(f) and output impedance ZL(f) of an amplification device having frequency characteristics, such as a transistor, and the input-output impedance Z0 of the peripheral circuit (system impedance) in each frequency band.
A conventional structure of a multiband matching circuit will be described below. The matching circuit is used in amplifiers. The same idea can be applied to matching circuits to be used in other equipment.
In one structure of the multiband matching circuit, switches or variable-capacitance elements are used to change a circuit constant. For example, a matching circuit 300 disclosed in Atsushi Fukuda, et al., “Multi-band Power Amplifier Employing MEMS Switches,” the Institute of Electronics, Information and Communication Engineers, General Conference 2004, c-2-4, p. 39, includes a main matching block 310, a delay circuit 321 having one end connected to the main matching block 310, a switch 322, and a sub matching block 323 connected through the switch 322 to the other end of the delay circuit 321, as shown in FIG. 16.
The matching circuit 300 is provided for signals in two frequency bands, and the central frequencies are denoted by f1 and f2, as shown in FIG. 17. When the matching circuit 300 is disposed between a circuit element 20 having ZI(f) and a port P1 to which a circuit of a system having a predetermined system impedance Z0, such as 50Ω or 75Ω, is connected, the matching circuit 300 is the circuit that establishes matching between ZI(f1) and Z0 and between ZI(f2) and Z0. The principle of matching will be described below.
If a first frequency band b1 (central frequency f1) is the operating band, the switch 322 is turned off (non conducting state). In this state, the main matching block 310 converts ZI(f1) to Z0. If the impedance of the delay circuit 321 in b1 is matched to Z0, the whole matching circuit can achieve matching with respect to signals in b1. The delay circuit 321 shown here as an example is a transmission line having a characteristic impedance Z0 in b1.
If a second frequency band b2 (central frequency f2) is the operating band, the switch 322 is turned on (conducting state). In this state, the main matching block 310 converts ZI(f2) to Z(f2). Generally, Z(f2) does not equal Z0. No matter what value Z(f2) is, by specifying the delay amount of the delay circuit 321 (line length, if a transmission line is used) and the reactance value of the sub matching block 323 connected via the switch 322 in accordance with single stub matching scheme, the impedance seen from the terminal P1 to the terminal P2 of the matching circuit 300 can be brought to Z0, because of single stub matching. In other words, the whole matching circuit can establish matching scheme also with respect to signals in b2. As for ZL(f), matching can be established by the same principle.
The matching circuit 300 implements matching in the two frequency bands by connecting and disconnecting the sub matching block 323 by turning on and off the switch 322. Instead of the series connection of the switch 322 and the sub matching block 323, a reactance element or a reactance circuit may be used. In that case, the same effects as in the case in which the series connection of the switch 322 and the sub matching block 323 is employed can be obtained by appropriately specifying the reactance, so that the reactance is not provided in b1 but provided in b2.
When the multiband matching circuit 300 is connected to an input-output terminal of an amplification device, the multiband matching circuit 300 can serve as a multiband amplifier that operates as an amplifier to signals in b1 when the switch 322 of the matching circuit 300 is in the off state and operates as an amplifier to signals in b2 when the switch 322 is in the on state.